1. Field of Invention Application
The present invention relates to an apparatus and a process for washing wafers and more particularly to an apparatus and a process for washing semiconductor wafers.
2. Description of the Related Art
Recently, a production of electronic devices on a semiconductor wafer has required for removing finer particulates as sizes of the semiconductor devices have been reduced. In general, the back surface of a semiconductor wafer has a great number of particulates stuck thereto during transport, hold or processing of the semiconductor wafer by a semiconductor producing apparatus. It is said that the number of these particulates are a few ten times to a few thousand times greater than the number of particulates sticking on the front surface of the semiconductor wafer. The particulates sticking to the back surface of the semiconductor wafer pass along an interface of a washing liquid and the semiconductor wafer and pollute the front surface of a preceding adjacent semiconductor wafer which has been immersed in the washing liquid when the next semiconductor wafer the back surface of which has the great number of particulates is immersed in the washing liquid during a wet process such as washing. Thus there is a problem that the particulates on the back surface of the semiconductor wafer reduce the yield of electronic devices formed on the front surface of the semiconductor wafer.
An example of a prior-art apparatus for washing a semiconductor wafer will be described with reference to FIG. 4, hereinafter. FIG. 4 illustrates the prior-art apparatus for washing a semiconductor wafer in which a wafer cassette holding semiconductor wafers is placed.
The apparatus comprises an internal wafer-washing tank 101, a washing-liquid filter 104, a circulating pump 105 and an external tank 111. The wafer-washing tank 101 receives a wafer cassette 107 holding semiconductor wafers 106. The external tank 111 surrounds the wafer-washing tank 101. The bottom of the external tank 111 has a washing-liquid outlet 110 communicating with the inlet of the circulating pump 105 through a pipe 103. The outlet of the circulating pump 105 communicates with the washing-liquid filter 104 which is connected to a washing-liquid inlet 109 which is provided in the bottom of the wafer-washing tank 101.
In operation, the circulating pump 105 delivers a washing liquid into the wafer-washing tank 101 through the washing-liquid inlet 109. The external tank 111 receives an overflow of the washing-liquid from the wafer-washing tank 101. The circulating pump 105 circulates the washing liquid from the washing-liquid outlet 110 to the washing-liquid inlet 109 through the washing-liquid filter 104. The washing-liquid filter 104 filters particulates 108 out of the washing-liquid from the washing-liquid outlet 110. Then, the circulating pump 105 delivers the purified or particulate-free washing-liquid from the washing-liquid filter 104 to the wafer-washing tank 101 through the washing-liquid inlet 109. Thus, the washing liquid passes upwardly through the interior of the wafer-washing tank 101. The wafer cassette 107 is immersed into or positioned in the washing liquid in the wafer-washing tank 101 so that the planes of the semiconductor wafers 106 held in the wafer cassette 107 are essentially perpendicular to the level of the washing liquid in the wafer-washing tank 101. Thus, there is a problem in that particulates 108 sticking to the back surfaces of the semiconductor wafers 106 may stick to the front surfaces of adjacent semiconductor wafers 106 in interfaces of the washing liquid and the semiconductor wafers 106 placed in the wafer-washing tank 101. There is also a problem in that free particulates 108 tend to stay on the surface of the washing liquid in the wafer-washing tank 101 since the washing liquid passes upwardly through the interior of the wafer-washing tank 101.
Japanese unexamined patent application publication HEI.2-132828 discloses a wafer-washing tank. This wafer-washing tank comprises a first vertical flow-straightening plate having a plurality of washing-liquid inlets, a second vertical flow-straightening plate having a plurality of washing-liquid outlets, the first and second vertical flow-straightening plates being arranged opposite each other through a spacing in which a wafer is placed, a washing-liquid horizontally flowing from the washing-liquid inlets to the washing-liquid outlets along the planes of the wafer. The wafer-washing tank lacks a means for horizontally holding the wafer in the washing-liquid in the wafer-washing tank in a case where the sidewall of the wafer-washing tank has a washing-liquid inlet. On the other hand, the sidewall of the wafer-washing tank lacks a washing-liquid inlet and a washing-liquid outlet but rather the bottom of the wafer-washing tank has the washing-liquid inlet in a case where the wafer-washing tank includes the means for horizontally holding the wafer in the washing-liquid in the wafer-washing tank. The publication HEI.2-132828 discloses no case where the sidewall of the wafer-washing tank has the washing-liquid outlet.